This invention relates generally to the field of rigid disc drives, and more particularly, but not by way of limitation, to a combination glide test and burnishing head, which can be utilized to facilitate manufacture of rigid magnetic recording media with extremely smooth surface characteristics.
Disc drives of the type known as xe2x80x9cWinchesterxe2x80x9d disc drives or rigid disc drives are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorenz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the xe2x80x9cfull height, 5 xc2xcxe2x80x3 format. Disc drives of the current generation typically have a data capacity of several gigabytes in a 3 xc2xdxe2x80x3 package, which is only one fourth the size of the full height, 5 xc2xcxe2x80x3 format or less. Even smaller standard physical disc drive package formats, such as 2 xc2xdxe2x80x3 and 5 xc2xcxe2x80x3, have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with xe2x80x9cparticulatexe2x80x9d recording layers. That is, small particles of ferrous oxide were suspended in a nonmagnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches, 12xcexcxe2x80x3) or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 1.0xcexcxe2x80x3, and products currently under development will reduce this flying height to 0.5xcexcxe2x80x3 or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a xe2x80x9cglidexe2x80x9d test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs a precision spin stand and a specially configured glide test head including a piezo-electric sensing element, usually comprised of lead-zirconium-titanate (PbZrTi3), also commonly known as a xe2x80x9cpzt glide test headxe2x80x9d. The glide test is performed with the pzt glide test head flown at approximately half the flying height at which the operational read/write head will fly in the finished disc drive product. For instance, if the disc being glide tested is intended for inclusion in a disc drive in which the operational heads will fly at 1.0xcexcxe2x80x3, the glide test will typically be performed with the pzt glide test head flying at 0.5xcexcxe2x80x3. If the glide test is completed without sensing any surface defects, then the disc is passed on the assumption that there will be no adverse effects on the operational heads and the discs during normal operation with a nominal head flying height twice that of the pzt glide test head flying height.
If, however, surface asperities or defects exist on the surface of the disc under test, the passage of the glide test head over the surface asperity will result in excitation of the glide test head, due to either direct contact between the pzt glide test head and the surface defect, or the disruption of the nominal hydrodynamic relationship between the rotating disc and the pzt test head. Current glide test head technology allows for the detection of media surface defects in the sub-microinch range.
When surface defects are detected on a disc, the disc is subjected to an additional manufacturing step called xe2x80x9cburnishingxe2x80x9d. Burnishing is accomplished through the use of specially configured burnishing heads. The burnishing head is engaged with a rotating disc and contact between the burnishing head and surface defects results in mechanical removal of the surface defects. Following the burnishing process, the glide test is again performed. Current economic considerations dictate that any given disc will be subjected to the burnishing and glide test processes only a limited number of times, such as twice, beforexe2x80x94in the continuing presence of surface defectsxe2x80x94being finally rejected.
Currently utilized glide test systems include a glide test head actuator which steps the glide test head across the data recording region of the disc being tested in small enough increments to ensure that the entire data recording surface passes beneath the glide test head. Should a surface asperity be detected during the glide test, the disc is burnished using a separate burnishing head. In order to minimize handling of the discs, it is common practice to provide a separate burnishing head actuator on the test system to move the burnishing head across the disc.
Once the burnishing head has been moved across the entire disc surface, the burnishing head is disengaged from the disc and the glide test is re-engaged with the disc and the glide test is re-performed. If the glide test is successful, the disc is passed to the drive manufacturing process. Should the glide test again detect a surface defect on the disc, the burnishing process is repeated.
Once again, it should be noted that the glide test/burnish processes are typically repeated only a limited number of times before the disc is scrapped as defective.
One of skill in the art will readily appreciate that prior art glide test/burnish systems which require separate actuators for glide test and burnishing heads include substantially duplicate mechanical and electronic components for the separate glide test and burnishing operations. A glide test/burnish system which utilizes a single common combination glide test/burnishing head assemblyxe2x80x94and thus a single mechanical actuator systemxe2x80x94for both the glide test and burnishing functions would result in significant reduction in capital equipment costs.
It is toward such a combination glide test/burnishing head and a system which utilizes such a combination glide test/burnishing head that the present invention is directed.
The present invention is a combination glide test/burnishing head which can be utilized in a glide test/burnishing system. The combination glide test/burnishing head includes two piezo-electric elements, which can be utilized in a passive mode as sensors for detecting contacts between the glide test/burnishing head and asperities on the surface of a magnetic recording disc. Contact between the glide test/burnishing head and disc asperities results in generation of a defect detection signal, which can be utilized by associated test logic to define the location of the detected asperities on the disc surface. The piezo-electric elements of the glide test/burnishing head can also be utilized in an active mode to cause yaw variation in the flight attitude of the glide test/burnishing head, in turn causing a burnishing pad on the glide test/burnishing head to be moved radially into contact with a detected disc asperity. Once an active, burnishing operation has been performed, the piezo-electric elements of the glide test/burnishing head are returned to passive mode, to determine if the burnishing operation was successful in removing the asperity on the disc surface. Combining the glide test and burnishing functions in a common head assembly allows the glide test and burnishing functions to be performed using a single actuator for the glide test/burnishing head, simplifying and reducing the cost of a glide test/burnishing system.